The present invention relates to voltage insulator appliances and particularly to high-voltage insulator appliances which are exposed to the outer atmosphere.
The terms "insulator", "insulator appliance", "electrical insulator" and "electrical insulator appliance" as used herein designate an appliance used to insulate a conductor from earth, or from another conductor and frequently also serving to support the conductor (Chambers Technical Dictionary, MacMillan Co., N.Y. 3rd Edition 1959, page 451 and Van Nostrands Scientific Encyclopedia, D. Van Nostrand Co., N.Y., 3rd Edition, 1958 pages 569-570).
It has been proposed to make such insulators, which should be weather-resistant even when exposed to high-voltage currents, of synthetic resins. Particularly, cycloaliphatic epoxy resins were proposed for this purpose which resins, preferably, were to be hardened with cycloaliphatic reactants. Unfortunately, however, such cycloaliphatic epoxy resins are only difficultly obtainable and thus relatively expensive, and thus, for economic and technical reasons, these cycloaliphatic epoxy resins do not fulfill the growing requirements for weather-resistant high-voltage insulators.
It has been proposed therefore to produce such insulators of two layers, namely so that the core portion was to be produced of relatively inexpensive synthetic resins which need not be as weather-resistant as the portion of the insulator appliance which actually is exposed to the outer atmosphere, and of an outer layer formed of a weather resistant, possibly fiber-reinforced, synthetic resin. Again, cycloaliphatic epoxy resins were proposed for forming the outer, exposed portion of such insulators.
However, it has since been found that the weather resistance of the conventional cycloaliphatic epoxy resins, when exposed to high voltage, does not meet the requirements in such cases. As is known under the influence of a creep current or an electric arc, conductive residues may be formed. An electric arc may be generated due to the deposition of a conductive dust layer formed at the surface of the insulator on exposure to industrial gases or ocean climate. This process may be simulated in a standard test procedure in the laboratory by repeatedly applying a thin graphite layer with a pencil to the surface of the insulator appliance and then burning off the graphite layer by applying a voltage thereto. The test is considered successful if the electric arc which is formed is extinguished in spite of even repeated exposure to the current and the voltage differential applied is maintained.
This property which indicates the weather resistance of the insulator appliance may be termed "creep current resistance regarding an extraneously formed layer".
It is therefore an object of the present invention to provide a weather-resistant high-voltage insulator appliance which will possess a high degree of creep current resistance.